Most of our attention is being directed towards obtaining further evidence for our hypothesis that transplantation immunity is major histocompatibility complex (MHC) restricted. Thus, we contend that transplantation antigens are not recognized per se but are only recognized in association with the MHC of the cells that present them to the immune system. We also propose that the direct effect of transplantation antigens on the immune system of the host is to induce tolerance, and that immunity occurs only when these antigens are presented to the host by antigen presenting cells (APC) MHC compatible with the graft. To further explore this situation we are turning our attention to the diabetes that occurs in BB rats. In this regard, attempts are being made to alleviate this hyperglycemic condition in suitably tolerant animals (rendered tolerant at birth by intravenous inoculation of bone marrow cells) with MHC compatible and incompatible islets of Langerhans. These islets are being transplanted under the kidney capsule or are being inoculated intraportally. (CS)